Design and assembly of the 117-kb <i>Phaeodactylum tricornutum</i> chloroplast genome
E. Walker, Mark Pampuch, Nelson Chang, Ryan R. Cochrane, Bogumil J. Karas
Abstract
There is growing impetus to expand the repertoire of chassis available to synthetic biologists. Chloroplast genomes present an interesting alternative for engineering photosynthetic eukaryotes; however, development of the chloroplast as a synthetic biology chassis has been limited by a lack of efficient techniques for whole-genome cloning and engineering. Here, we demonstrate two approaches for cloning the 117-kb Phaeodactylum tricornutum chloroplast genome that have 90% to 100% efficiency when screening as few as 10 yeast (Saccharomyces cerevisiae) colonies following yeast assembly. The first method reconstitutes the genome from PCR-amplified fragments, whereas the second method involves precloning these fragments into individual plasmids from which they can later be released. In both cases, overlapping fragments of the chloroplast genome and a cloning vector are homologously recombined into a singular contig through yeast assembly. The cloned chloroplast genome can be stably maintained and propagated within Escherichia coli, which provides an exciting opportunity for engineering a delivery mechanism for bringing DNA directly to the algal chloroplast. Also, one of the cloned genomes was designed to contain a single SapI site within the yeast URA3 (coding for orotidine-5'-phosphate decarboxylase) open-reading frame, which can be used to linearize the genome and integrate designer cassettes via golden-gate cloning or further iterations of yeast assembly. The methods presented here could be extrapolated to other species-particularly those with a similar chloroplast genome size and architecture (e.g. Thalassiosira pseudonana).